1. Field of the Invention
The present invention relates to a lens system, and more particularly to a temperature-compensated lens system comprising a plastic lens and a liquid lens for compensating variations in temperature at the image forming plane of the lens system.
2. Background of the Invention
Most conventional lens systems comprise a plurality of lenses which are all made of glass. However, more and more plastic lenses are used since lenses are often required to have aspherical surfaces to reduce aberration and also since they need to be smaller in weight and manufactured at less cost. There are now put to use both plastic lens systems with all of their lenses made of plastic and also hybrid lens systems including both glass and plastic lenses. The plastic lens systems are however disadvantageous in that the image forming plane tends to be significantly displaced due to a temperature-dependent variation in the refractive indices of the plastic lenses which puts the image out of focus. While the hybrid lens systems are devised to improve the temperature characteristics of lens systems, they still fail to provide fully satisfactory performance because the refractive indices of the plastic lenses with their refracting surfaces involved in establishing the lens power are subject to temperature-dependent variations.
One hybrid lens system of large size which has heretofore been employed in a projection-type television display will be described with reference to FIG. 1 of the accompanying drawings. A light beam K.sub.2 emitted from an electron gun CRT (cathode ray tube) shown at the right hand end passes through a sealed glass sheet L.sub.5 in the electron gun CRT, a cooling liquid E.sub.2 cooled by a liquid of the electron gun CRT and serving as a liquid lens, a lens L.sub.4 of plastic which seals the cooling liquid E.sub.2, a convex lens L.sub.3 of plastic having an aspherical surface S.sub.6, a convex lens L.sub.2 of glass having the maximum power of the lens system, and a plastic lens L.sub.1. The thus transmitted light beam K.sub.2 falls on a large-size projection screen S for displaying an image as it is scanned by the light beam K.sub.2 from the electron gun CRT. The light beam K.sub.2, indicated by the dotted line, which is emitted from the electron gun CRT is refracted by the surface S.sub.3 of the lens L.sub.2 and also by the surfaces S.sub.2 and S.sub.1 of the lens L.sub.1 before it reaches the screen S. Denoted at S.sub.1 through S.sub.9 and S.sub.20 are lens surfaces. The lens surfaces S.sub.2, S.sub.6 and S.sub.7 are aspherical for correcting aberration.
With the conventional lens system arrangement, however, temperature-dependent variations in the refractive indices and focal lengths of the lenses, particularly temperature-dependent variations in the total power of the lens system, cannot be eliminated. Hence, the lens system fails to provide high resolution, brightness, and clear profiles for high image quality.
Where glass lenses suffering small temperature-dependent variations in refractive indices are employed, the above problems can be avoided to a certain extent. However, the glass lenses are heavy, cannot be ground with ease for producing aspherical surfaces, and are highly expensive. Consequently, most conventional lens systems of the type in question are of the hybrid type in which only a convex lens of positive power, such as lens L.sub.2 in the lens system shown in FIG. 1, is made of glass.
Although plastic lenses are lightweight, can be ground with ease for aspherical surfaces, and are less costly, the lens system should not be made entirely of plastic material inasmuch as temperature-dependent variations in refractive indices of plastic lenses are large.
Where a hybrid lens system is employed, since the refracting surfaces of the plastic lenses are responsible for establishing the lens power, images formed on the screen may be out of focus due to temperature-dependent variations in the refractive indices of the plastic lenses.